The invention relates to a process for displacing oil within a permeable material such as a subterranean reservoir. It is useful for displacing reservoir oil toward a location from which oil is produced, for displacing residual oil away from a well to improve injectivity, or the like.
Aqueous anionic surfactant systems are known to be particularly efficient for displacing oil. Such an anionic surfactant system is a substantially homogeneous aqueous liquid composition that may comprise a solution, a microemulsion, or a micellar dispersion of anionic surfactant molecules and/or micelles. The water-solubilities and oil-solubilities of the surfactants in such a system are such that those materials tend to remain along an oil/water interface, rather than being completely dissolved or dispersed within either the water-phase or oil-phase of the system. The anionic surfactants comprise surface active salts or soaps of organic or inorganic acids.
In a process for displacing oil, the surfactants in an aqueous anionic surfactant system can be pre-formed or formed within a permeable material such as a subterranean reservoir. The J. Reisberg U.S. Pat. No. 3,174,542 describes oil displacing processes in which acidic materials are injected ahead of basic materials so that surfactant systems are formed in situ. Pre-formed active aqueous petroleum sulfonate surfactant systems containing activity-enhancing concentrations of electrolytes are described in the J. Reisberg U.S. Pat. Nos. 3,330,344 and 3,348,611. Active aqueous anionic surfactant systems containing electrolytes, water thickening material, and chemicals selected for their ability to favorably modify emulsions formed between the oil and the oil-displacing surfactant system are described in the H. J. Hill and D. R. Thigpen U.S. Pat. Nos. 3,712,377 and 3,768,560.
Although aqueous anionic surfactant systems are generally efficient oil-displacing fluids they have relatively low tolerances to multivalent cations. Such cations are commonly encountered in waters or brines in subterranean reservoirs that contain solutions of calcium or magnesium salts, or the like. The multivalent cations tend to react with the anionic surfactants or the components of anionic surfactant systems to form precipitates, to cause phase separations, or the like.
Multivalent cation tolerance problems typical of aqueous anionic surfactant systems are discussed in patents such as the J. Reisberg, J. B. Lawson and G. Smith U.S. Pat. No. 3,508,612; the R. F. Farmer, J. B. Lawson and W. M. Sawyer, Jr. U.S. Pat. No. 3,675,716; etc. Those patents suggested improving the multivalent cation tolerance by dissolving co-surfactants, such as alkoxy-alcohol sulfates in the systems. U.S. Pat. No. 3,799,264 suggests using surfactant systems that contain sulfated aliphatic anionic surfactants, nonionic surfactants, and a water-soluble guanidine salt, in order to provide an improved tolerance to multivalent cations.